New York--Ad buys spread out across several online networks are more efficient than buys placed on fewer networks, according to a report from Jupiter Media Metrix Inc. Jupiter found that placing a four-week ad campaign of 4 million impressions on any one of the leading four Web sites would reach an audience of 1.4 million to 1.9 million unique visitors. Spreading out the same 4 million impressions across three of the top four sites increased the audience to between 2.2 million and 2.5 million unique visitors, the study found. âThe reality on the Internet is that target audiences are often fragmented across multiple sites, thereby limiting the ability of one or just a few sites to efficiently reach a greater number of defined audience members,â said Mainak Mazumdar, head of research and product management for Media Metrix. âConversely, advertisers looking to increase frequency in a campaign may very well lean toward advertising on fewer sites.â